Commercial high performance Time of Flight mass spectrometers generally utilise ion detection systems comprising microchannel plates for pre-amplifying ion pulse signals. Microchannel plates generate multiple electrons in response to an ion striking the input surface of the microchannel plate. The electrons which are generated by the microchannel plate provide an amplified signal which may then be subsequently recorded using a fast Analogue to Digital Converter (“ADC”) or a Time to Digital Converter (“TDC”). Ion detectors comprising two microchannel plates are advantageously used for amplification of ion pulse signals in Time of Flight mass spectrometers.
Microchannel plate ion detectors are particularly advantageous for use in Time of Flight mass spectrometers since they provide a high gain amplification. For example, a single ion striking the input surface of a microchannel plate ion detector will typically cause several million electrons to be emitted from the output surface of the microchannel plate which can then be recorded. Microchannel plate ion detectors also have a relatively fast response time. Typically, an ion striking the input surface of a microchannel plate ion detector will generate a pulse of electrons having a pulse width of the order of a few nanoseconds at half pulse height. A further advantage of microchannel plate ion detectors is that the input surface of the microchannel plate is relatively flat and hence ions travel a relatively constant distance to the microchannel plate. Therefore, any spread in the arrival times of the ions at the input surface of the microchannel plate(s) is effectively negligible.
Although conventional microchannel plate ion detectors have several advantages they also have several disadvantages. In particular, conventional microchannel plate ion detectors suffer from signal induced ringing noise and/or reduced bandwidth caused by impedance mismatching between the collection anode which collects electrons from the microchannel plate(s) and the 50 Ω input amplifier of the Analogue to Digital Converter or the Time to Digital Converter used as part of the acquisition electronics. Another disadvantage of conventional microchannel plate ion detectors results from the requirement that Time of Flight mass spectrometers are designed to mass analyse ions having relatively high kinetic energies, typically several keV. In order to achieve such relatively high ion kinetic energies the ions are normally accelerated through an electric field generated by a high voltage difference between the ion source and the field free drift tube of the Time of Flight mass analyser. The mass spectrometer may be configured, for example, such that the ion source is floated at a high voltage and the flight tube is grounded or vice versa. However, normally the input amplifier of an Analogue to Digital Converter or a Time to Digital Converter in the ion detector is required to be operated at ground potential. Therefore, in order to apply an appropriate bias voltage to accelerate the electrons from the microchannel plate(s) to the collection anode of the ion detector it may be necessary to capacitively decouple the collection anode from the input of the Analogue to Digital Converter or the Time to Digital Converter. However, conventional approaches to capacitively decoupling the collection anode from the Analogue to Digital Converter or the Time to Digital Converter cause impedance mismatching between the collection anode and the Analogue to Digital Converter or the Time to Digital Converter. A further disadvantage of conventional microchannel plate ion detectors is that the collection anode tends to capacitively pick up high frequency noise from nearby circuitry such as high voltage power supplies which are used to power the microchannel plate(s) or the collection anode.
The combined effects of signal induced ringing noise, reduced bandwidth and high frequency noise pick-up in conventional microchannel plate ion detectors are detrimental to the mass resolving power and detection limits of the overall Time of Flight mass spectrometer. A further disadvantage of conventional microchannel plate ion detectors is that signal saturation may result from electron depletion in the microchannel plate(s) immediately after a relatively large ion pulse has been detected. This signal saturation results in a reduction of gain of the ion detector immediately after detection of a relatively large ion pulse.
It is therefore further desired to provide an improved microchannel plate ion detector.